We did AK & MK last year. Photographer did ask if we had any specific places in mind, so try to think of locations ahead of time. Your session will go by very fast, and you need to take the time to walk to the different locations.
As a PP recommended, be ready to go (makeup, ears, props, etc.) when your time slot starts. Plan your attire ahead of time. (DD was celebrating her graduation so we had Graduation ears & a graduation t-shirt, but also a change of ears & t-shirt ready to go.) Also, if possible, think of the time of day you are picking (summer has the afternoon rain & the heat of the day or nap times for younger children)
For AK we let him pick the best locations & the pics were wonderful. Some on the path near the tree of life. Really beautiful shots.
For MK - our only request was Castle shots, the rest we left to him. Again, perfect pictures - some in the ally on Main Street, others near the Castle. Thankfully I was able to book at pre-park time slot, so the Castle shots, we were on the pathway & under the 50th sign with no one around. The photographer was EXCELLENT!! (And happened to be the same for both days. )
Looking forward to reading about anyone who has sessions at Epcot!